1. Field of the Invention
The present invention, which belongs to the field of electroacoustic transducers and mechanical-electrical transducers in electricity, relates to a transducer and in particular to an inner magnetic transducer with multiple magnetic gaps and multiple coils.
2. Background of the Related Art
During one-hundred-thirty years since the world's first moving-coil speaker (hereinafter referred to as speaker) was granted for a patent for invention in 1877, almost all the speakers of commercial production are equipped with only one magnetic gap and one coil, except those as disclosed by the invention patents of U.S. Pat. No. 5,849,760 in the name of USA HARMAN Company, CN951010204 in the name of JAPAN ALPINE Company, and CN99114781.2, CN00122197.3, U.S. Pat. No. 6,795,564 and TW88109796 in the name of the present inventor and the like. When the coil is connected to an audio signal current, a left-handed electrodynamic force F is generated under interaction with the magnetic field of the magnetic gap according to the Fleming's left-hand rule, such that the coil and the vibrating membrane is driven to take piston-like reciprocation and sounds are produced due to air vibration. However, at the same time when the coil reciprocates like a piston, according to the Fleming's right-hand rule, the permanent magnetic lines within the magnetic gap will cut perpendicularly the coil to thereby induce an electric generator potential within the same coil, namely, a so-called back electromotive force of a speaker by the electroacoustic technical field. As the vector of the back electromotive force has a difference in phase angle of 180 degrees with the vector of the audio input signal (ignoring the inductance and wire-to-wire capacitance of the coil), they are superposed within the same coil, and the back electromotive force necessarily incurs distortion during electroacoustic restoration of the speaker. Obviously, it is an undesirable yet inextricable physical phenomenon.
Generally, the greater the relatively moving speed, the relatively moving range and the inductance of the coil of the speaker are, the lower the frequency of the audio signal current while the larger the amplitude of the back electromotive force become, with a result that a greater distortion is incurred. Thus, it is impossible or unwilling for the prior art to improve the sensitivity of the speaker, i.e. the electroacoustic conversion efficiency, so as to prevent the troublesome back electromotive force from incurring serious distortion. Even the back electromotive force of a professional speaker when being operated under a big, high-powered dynamic signal, may breakdown and destroy the power amplifier tube at the final stage of the power amplifier.
During one hundred and thirty years, in face of such a worldwide technical difficulty remaining unsolved in the electroacoustic field, people can but choose a passive technical solution to fetch up this difficulty, namely, reducing the electroacoustic conversion efficiency (i.e. sensitivity) as much as possible on one hand, and meanwhile enhancing the input power of the speaker as much as possible on the other hand, such that the ratio between the absolute value of the back electromotive force and the input audio signal is decreased to thereby limit the distortion caused by the back electromotive force at a commonly acceptable level. It is why some Hi-Fi speakers, even some Hi-end speakers under world-famous trademarks, cannot be promoted by power amplifiers.
Secondly, the other fatal drawback of the transducer with only one magnetic gap and only one coil is high heat generation incurred by a low efficiency.
Even if the adverse factor of back electromotive force is eliminated, the lower efficiency of the speaker is a further worldwide technical deficiency which puzzles the electroacoustic field for a long period of one hundred and thirty years.
For example, a speaker of 2-inch caliber has the electroacoustic conversion efficiency equal to or less than 0.10%. This means that, when the speaker is inputted with an audio power of 5 W, only the electric energy of 0.005 w is converted into the desired sound energy, and the rest 4.99 W is converted into useless and harmful heat and wasted. In this case, the efficiency of the speaker of 2-inch caliber is equivalent to about 1/70-1/80 of the efficiency of an incandescent lamp.
A professional speaker of 15-inch large caliber usually has a SPL-value of 98 dB/1 W/1 m, whose efficiency is 3.89%, less than one half of the efficiency of the incandescent lamp. There are hundreds of billions of speakers in the modern society, and almost all of them are operated at super low efficiencies (only except the “speakers” in the field of thermoacoustic refrigeration). They waste a large amount of energy of the human society, while increase the discharge amount of carbon dioxide drastically.
The third drawback of the transducer with a single magnetic gap and a single coil is that, due to the T-iron structure of the magnetic path, the narrow magnetic gap has a sealed back chamber at the bottom. When the coil reciprocates like a piston in the magnetic gap, the air accumulated in the back chamber forms an airbag damping against the coil, which deteriorates the instantaneous response of the speaker, which enhances the distortion of the speaker, and which decreases the fidelity in the course of electroacoustic restoration. Meanwhile, at the intersection of the T-iron and the lower polar plate, the flux density of the magnetic lines has already been saturated yet is far from being sufficiently utilized, rendering a further waste of the magnetic energy.
The fourth significant drawback of the transducer with a single magnetic gap and a single coil is that, generally, it is impossible to attain a full-range electroacoustic restoration effect by use of only one speaker. It is because that, the speaker has an inductance, whose impedance value is presented as a function of the audio current working efficiency. The lower said efficiency is, the lower the impedance becomes, and vice versa. That is to say, for every traditional speaker, the effective value of the high audio current flowing through the coil is much less than that of the low frequency audio current. Hence, the sound pressure generated by the speaker when being operated at the high audio section obviously decreases with respect to the low audio section. Of course, for a speaker with a caliber no more than 3-inch, it is possible to fetch up this deficiency by technical means as the vibrating quality of the system is relatively light. However, a speaker of relatively small caliber must have a relatively higher Fo, and thus it is hard for the speaker to produce a satisfactory low audio effect. For a speaker with a caliber no less than 3-inch, Fo tends to the low audio section as a result of the increased caliber, such that the low audio electroacoustic restoration quality of the speaker is improved. However, along with the increased caliber, the quality of the vibrating system increases synchronously, rendering a great decrease in the audio section of 5-10 KHz for the output sound pressure in the high audio section of the speaker.
For this reason, people have to assemble a bass speaker, a middle speaker and a high speaker into a speaker system by a crossover network so as to attain a relatively satisfactory electroacoustic restoration effect. However, the introduction of the crossover network not only accelerates the consumption of electric energy, but also brings forth new higher harmonic restorations within those frequency bands near the crossover points.
Hence, people try to change the structure mode of a traditional speaker with a single magnetic gap and a single coil, and several new technical solutions about a transducer with multiple magnetic gaps and multiple coils have been proposed.
For example, in the prior art, USA HARMAN Company has proposed, in its invention patent U.S. Pat. No. 5,748,760 (PCT/US95/14696, WO96/33592), a driver for (a transducer with) double magnetic gaps and double coils using a multifunctional frame. However, it has the following drawbacks: first of all, the front polar plate, the rear polar plate and the neodymium magnet are provided with central axial holes. For the transducers of mini-type and the speakers with small or middle calibers, the effective dimension of the neodymium magnet and its magnetic energy are subjected to unreasonable restraints. Therefore, the invention is unavailable for the widely-applied series of speakers of mini-type and speakers with small or middle calibers. Secondly, the patent U.S. Pat. No. 5,748,760 is silent to essential contents necessary for attaining a transducer possessed of resistance load characteristics. Thirdly, this patent, when applied in high-power transducers, arranges center plats at the central axial holes of the polar plates and the neodymium magnet for guiding the coil's wire out. This finally loses a direct pneumatic heat-dissipating passage for huge heat generated by the transducer. Fourthly, the electroacoustic conversion efficiencies of the products with the patent U.S. Pat. No. 5,748,760 (for example, the speaker units in EON voice boxes from USA JBL Company) are not significantly improved in comparison with those traditional speakers using the iron strontium oxide.
UK NXT Company has also proposed, in its patent application PCT/GB00/01484 (CN1347628A), a driver for an inner magnetic transducer with multiple magnetic gaps and multiple coils. However, it has the following drawbacks: the patent application fails to disclose the entire technical solution for forming a driver with multiple magnetic gaps and multiple coils. Moreover, it falls into the range covered by the claims of the patent CN2333135Y, and the inventor's patent CN97205593.2 and patent applications PCT/CN98/00306 (WO99/31931) and CN1219834A. In addition, the method of calculating the coil inductance in the speaker with a permanent magnet and an iron-core circuit by use of the Welsby's formula, and its corresponding conclusion, as raised by the above patent application, cannot be established.
The present inventor, in his patent CN200520035371.X and patent applications PCT/CN98/00306, CN99114781.2, US2005/0099255 and CN1741683A, has also proposed several kinds of inner magnetic transducers which have multiple coils and multiple magnetic gaps, and which are possessed of resistance load characteristics, or characteristics similar thereto. However, they have the following drawbacks: first of all, these technical solutions fail to give full definitions upon the technical features in the symmetrical magnetic paths and symmetrical coil circuits of the transducers. Secondly, the bracket made of non-magnetic material encloses the entire magnetic paths from upside to downside, which inevitably increases the weight, the complex of the entire structure and the production cost of the transducer. Thirdly, the patent applications fail to give necessary, sufficient disclosure and description upon how the back electromotive back is eliminated from the transducer. Fourthly, when the two end surfaces of the annular magnetic yoke are flush with the outer polar surfaces of the upper and lower polar plates, as shown in FIG. 12, it is inevitable to increase the asymmetry of the magnetic path and thus enhance the distortion of the transducer accordingly.
JAPAN SONY Company, in its patent application JP2006050245 (CN1735282, US2006029238, DE102005036538), has proposed an apparatus and a method for eliminating the back electromotive force in a transducer. However, in order to eliminate the signal distortion caused by the back electromotive force, it has to add a distortion rectifying circuit consisting of three electronic amplifiers for every speaker.